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Abbreviation for tera (1012). See International System of Units.

T junction

See series T junction.

T reference point

In Integrated Services Digital Networks (ISDN), the conceptual point dividing NT2 and NT1 functional groupings in a particular ISDN arrangement.


The generic designator for any of several digitally multiplexed telecommunications carrier systems.
Note 1: The designators for T-carrier in the North American digital hierarchy correspond to the designators for the digital signal (DS) level hierarchy. See the associated table below.
Note 2: T-carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission.
Note 3: If an “F” precedes the “T”, a fiber optic cable system is indicated at the same rates.
Note 4: The table below lists the designators and rates for current T-Carrier systems.
Note 5: The North American and Japanese hierarchies are based on multiplexing 24 voice-frequency channels and multiples thereof, whereas the European hierarchy is based on multiplexing 30 voice-frequency channels and multiples thereof. See table below.

T-Carrier Systems North American Japanese European (CEPT)
Level zero (Channel data rate) 64 kb/s (DS0) 64 kb/s 64 kb/s
First level 1.544 Mb/s (DS1) (24 user channels) 1.544Mb/s (24 user channels) 2.048 Mb/s (30 user channels)
(Intermediate level, North American Hierarchy only) 3.152 Mb/s (DS1C) (48 Ch.)
Second level 6.312 Mb/s (DS2) (96 Ch.) 6.312 Mb/s (96 Ch.), or 7.786 Mb/s (120 Ch.) 8.448 Mb/s (120 Ch.)
Third level 44.736 Mb/s (DS3) (672 Ch.) 32.064 Mb/s (480 Ch.) 34.368 Mb/s (480 Ch.)
Fourth level 274.176 Mb/s (DS4) (4032 Ch.) 97.728 Mb/s (1440 Ch.) 139.268 Mb/s (1920 Ch.)
Fifth level 400.352 Mb/s (5760 Ch.) 565.148 Mb/s (7680 Ch.) 565.148 Mb/s (7680 Ch.)

Note 1: The DS designations are used in connection with the North American hierarchy only.Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system that operates at 90 Mb/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development.


A passive optical coupler having three ports (three fibers). Note 1: Two isolated inputs may be combined into one output; or one input, into two isolated outputs. Note 2: The amount of coupling loss, usually expressed in dB, between ports is determined by the design and construction of the coupler. [After FAA] Synonyms directional coupler, splitter, tee coupler.


For basic rate access in an Integrated Services Digital Network (ISDN) environment, a user-to-network interface reference point that (a) is characterized by a four-wire, 144-kb/s (2B+D) user rate, (b) accommodates the link access and transport layer function in the ISDN architecture, (c) is located at the user premises, (d) is distance sensitive to the servicing network terminating equipment, and (e) functions in a manner analogous to that of the Channel Service Units (CSUs) and the Data Service Units (DSUs).


Transport – Multi Protocol Label Switching


Transport Stream System Target Decoder (MPEG-2)

T1 (carrier)

See T-carrier.

T1 line

A full-duplex digital transmission facility that is composed of transmission media (optical or metallic) and regenerators that carry one DS1 signal. [After T1.408-1990]

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